[RISCV] Add isel patterns for grevi, shfli, and unshfli to brev8/zip/unzip instructions.

[RISCV] Add isel patterns for grevi, shfli, and unshfli to brev8/zip/unzip instructions.

Zbkb supports some encodings of the general grevi, shfli, and
unshfli instructions legal, so we added separate instructions for
those encodings to improve the diagnostics for assembler and
disassembler. To be consistent we should always use these separate
instructions whenever those specific encodings of grevi/shfli/unshfli
occur. So this patch adds specific isel patterns to override the generic
isel patterns for these cases. Similar was done for rev8 and zext.h
for Zbb previously.